Color and substitution pattern in anthocyanidins. A combined quantum chemical-chemometrical study

The relationship between color and substitution patterns in anthocyanidins has been investigated with the aim of developing quantitative structure-col or models. Experimental data for the lowest UV transition in 20 substituted anthocyanidins are reviewed. The excitation energies are modeled by semi-e mpirical ZINDO/S calculations, both at the self-consistent field level of t heory and by expanding the excited state in a basis of singly excited confi gurations (CIS). Even though CIS is required to obtain the right order of m agnitude, a better correlation between observed and theoretical excitation energies is achieved through orbital energies according to Delta E = epsilo n(pi*) - epsilon(pi) - 3.37 eV. At this point, the agreement between theory and experiment is somewhat less than satisfactory. Next, the potential of employing theoretical and positional parameters in a multivariate regressio n model for the corresponding wavelength (lambda(max)) is explored in terms of the partial least squares (PLS) algorithm. The resulting models substan tially improve upon the quantum chemical estimates, and seem to provide hig h predictive power while keeping the computational cost low. The PLS models are used tb address the discrepancy between different experimental lambda( max) values for apigeninidin and luteolinidin reported in the literature. ( C) 1999 Elsevier Science B.V. All rights reserved.